Mid-level
dry intrusions have been mentioned as a factor in tornado outbreaks associated
with landfalling tropical cyclones for more than three decades, but a systematic
analysis of historical outbreak cases with respect to this pattern has
been missing. Herein, following an examination of all named tropical
cyclone landfalls from the Atlantic and Gulf of Mexico for the period 1960-1999,
thirteen “outbreak” cases (defined as storms producing twenty or more tornadoes)
have been identified and eleven of the cases offering clear evidence of
a dry intrusion at mid-levels over the outbreak area. The outbreaks all
occurred when the favored area for tornadogenesis (the right-front or northeast
quadrant of the storm) coincided with a pronounced gradient of relative
humidity, with the gradient best reflected at the 700 hPa and/or 500 hPa
levels. Two distinct patterns were identified with respect to the origin
of the mid-level dry air involved in these cases: in one, a mass of dry
air that impinged on much of the northern or northwestern semi-circle of
the storm's outer circulation gradually divided into two separate areas
(one to the northwest and the other to the northeast of the storm center)
as the storm advanced; the other involved ingestion of a lobe of dry air
from a reservoir most often located in the eastern semi-circle of the storm.
The outbreak cases were determined to have a strong diurnal signal detected
in the temporal distribution of the tornadoes, as almost two-thirds (65%)
of the tornadoes in the eleven "positive" cases occurred between 15 UTC
and 24 UTC (daylight hours in the region examined). A review of all tropical
cyclone landfall events since 1960 revealed that there are only three cases
with clear evidence of a mid-level dry intrusion which failed to produce
an outbreak as defined herein.

Notes: (a) Minor editorial variations may exist
between the published and this on-line version. In case of any such variations,
the published version is deemed to be the official source. (b) In the on-line
version, footnotes appear at the bottom of the section in which they occur.1. INTRODUCTION

The potential
for landfalling tropical cyclones (hereafter, TCs) to produce tornadoes
has been previously documented. Some storms produce tornado outbreaks while
others produce few or none. Vescio et al. (1996) noted that mid-level dry
intrusions have the potential to substantially alter the thermodynamic
structure of the TC environment (with significant enhancement of CAPE and
surface-based instability), and suggested it would be beneficial to document
future tornado outbreaks to better understand the role of dry intrusions
in the outbreaks. Examination of historical outbreak cases yields important
information and has the advantage of providing improved understanding of
the role of mid-level dry intrusions without awaiting future storms.

Thirteen “outbreak”
cases, defined herein as storms producing twenty or more tornadoes, for
TCs making landfall from the Atlantic and Gulf of Mexico have been examined
for evidence of mid-level dry intrusions. This definition is consistent
with the “large outbreak” definition found in Galway (1975, 1977) as applied
to all tornado outbreaks.¹ Eleven cases were found to offer
clear evidence of a dry intrusion at mid-levels over the outbreak area.
The outbreaks all occurred when the favored area for tornadogenesis (the
right-front or northeast quadrant of the storm) coincided with a pronounced
gradient of relative humidity, with the gradient best reflected at the
700 hPa and/or 500 hPa levels.

Two distinct
patterns were identified with respect to the origin of the mid-level dry
air involved in these cases. In one, a mass of dry air that impinged on
much of the northern or northwestern semi-circle of the storm's outer circulation
became divided into two lobes as the storm advanced, with one to the northwest
and the other to the northeast of the storm center. In the other, dry air
from a reservoir most often located in the eastern semi-circle of the storm
was ingested into the storm’s circulation. The eleven "positive" outbreak
cases had a strong diurnal signal detected in the temporal distribution
of the tornadoes; almost two-thirds (65%) of the tornadoes occurred between
15 UTC and 24 UTC (daylight in the region examined).

¹ McCaul (1991) classified
tropical cyclone tornado outbreaks as minor (not more than 8), major (9
to 23 tornadoes) or severe (more than 24 tornadoes). A review of McCaul’s
Table 1 (not shown) reveals that 124 tornadoes were produced by the eleven
storms producing 10 to 19 tornadoes; 278 tornadoes were produced by the
seven storms producing 20 or more tornadoes.

2. BACKGROUND

2.1 Factors in Tornado Location: Two areas
within TCs produce most of the reported tornadoes: near the core of the
storm and in the outer rainbands (Gentry 1983; Weiss 1987). Although Gentry
said that most tornadoes occur in areas close to the coastline and Novlan
and Gray (1974) asserted that nearly all hurricane-induced tornadoes occur
within 200 km of the coast, it is now clear that the threat can extend
well inland.

Novlan and
Gray (1974) found that the centroid of tornadogenesis is 240 km northeast
(i.e. 50 degrees east of due north) from the storm center. Gentry (1983)
noted that the tipping term² of the vorticity equation is twice as
large in the northeast quadrant of a landfalling storm. McCaul (1991) examined
1,296 proximity soundings from TCs producing tornadoes and found that ambient
helicity is maximized in the right-front quadrant. Bogner et al. (2000)
examined dropwindsonde data from six Atlantic storms while over open water
and found that the right-front quadrant (relative to storm motion) contained
the greatest speed and directional shear.

***

2.2 Factors Leading to Tornadogenesis:

Various factors
have been proposed to explain tornadogenesis in landfalling TCs. Novlan
and Gray (1974) noted that speed shear is pronounced in areas within 200
km of the coast as a storm makes landfall. They estimated a 20 m s-1 increase
in wind velocity from the surface to 850 hPa. Gentry (1983) noted that
surface wind speeds over land areas were reduced by friction while the
flow at slightly higher levels remained strong and was not affected by
friction.

Studies since
the deployment of the WSR-88D radars indicate that many of the tornadoes
are related to intense, persistent cells within the outer rainbands (Spratt
et al. 1997). These cells develop mesocyclones, although identifying the
mechanism by which this occurs has proven elusive. McCaul and Weisman (1996)
simulated shallow supercell storms in landfalling hurricane environments
and found that while the wind shear profiles often favor the development
of storms with rotating updrafts, low-level mesocyclogenesis may be hampered
by a relative lack of strong storm-induced baroclinity in the simulated
storms.

Meso-beta
scale boundaries can be an important factor in tornadogenesis associated
with supercellular storms (Markowski et al. 1998). TC outer rainband cells
that develop mesocyclones have been described as mini-supercells (Spratt
et al. 1997; McCaul and Weisman 1996) and tornadoes have been documented
as the cells in TC outer rainbands and other peripheral mini-supercells
interact with baroclinic boundaries (Edwards et al. 2000).

***

2.3 Significance of Mid-Level
Dry Intrusions: The traditional
TC paradigm is a system dominated by barotropic influences. The barotropic
structure of the typical TC appears to limit CAPE to levels well below
those normally associated with deep convective storms on the Great Plains
(McCaul 1991). In apparent contrast to McCaul’s findings, Bogner et al.
(2000) examined dropwindsonde data from six Atlantic hurricanes over open
ocean, and found CAPE of around 1500–1700 J kg-1 at 300 km or more from
the storm center and found little variation in CAPE by storm quadrant.

The origin
of baroclinity in an otherwise barotropic system has been the subject of
investigation, informed speculation, and some consternation for at least
the past four decades. Discussing convective instability as a “concomitant
ingredient” for tornado formation in hurricanes, Hill et al. (1966) noted
that the notion of relatively dry air at intermediate levels aloft is difficult
to reconcile with the generally held view that the TC consists of a roughly
uniform air mass both vertically and horizontally that is at or near saturation.

Novlan and
Gray (1974) specified “significant dry air intrusions in the right rear
quadrant … at 850-700 mb” as an indicator for potential tornado “family”
outbreaks. McCaul (1987) noted that dry air advected into a landfalling
storm’s mid-levels may influence the structure of the storm, and that mid-level
dry air intrusions are fed by a large-scale reservoir of dry
air outside the hurricane that may eventually penetrate quite close to
the center of a storm, causing local increases in the lapse rate, thereby
contributing to an increase in convective instability. Thus, the notion
that mid-level dry intrusions may occur in TCs is not of recent origin.

Cione et al.
(2000) examined the "conventional wisdom" that differences between sea
surface temperatures (SSTs) and surface air temperatures (TAs) within the
hurricane environment are small and do not vary much as a function of distance
from the storm center. Using 7,800 individual observations from the National
Data Buoy Center's moored and drifting buoys, they developed a database
of 153 time series from 37 hurricanes over a 23-year period (1975-1998).
They found a number of events where TAs were significantly cooler than
SSTs, and concluded that these appeared to be related to concurrent episodes
of low-level drying.

***

² The preferred terminology
is “twisting” term, but tilting term and tipping term are also used. The
process describes a term of the vorticity equation that represents generation
of vertical vorticity by the twisting of horizontal vorticity into the
vertical through the agency of shear in the vertical velocity. (Glickman,
2000)³ These over-water
values are of unknown relevance for TCs that have made landfall, with the
possibility of several factors altering the kinematic and thermodynamic
structure of the TC post-landfall.3. EXAMINATION OF OUTBREAK CASES

For purposes
of this study, outbreak cases are defined as those occurring subsequent
to 1960 and producing 20 or more tornadoes with landfall on the Atlantic
or Gulf coast (including the northern Mexican coast because of its proximity
to Texas). The storms meeting these criteria are: Agnes (1972), Alicia
(1983), Allen (1980), Andrew (1992), Beryl (1994), Beulah (1967), Carla
(1961), David (1979), Danny (1985), Georges (1998), Gilbert (1988), Josephine
(1996), and Opal (1995). Beryl and Josephine were tropical storms; all
others were hurricanes. Table 1 contains a list of the storms, the number
of significant tornadoes produced by each, and the casualties which resulted.

The location
of each tornado was ascertained utilizing SVRPLOT2 (Hart and Janish 1999),
Significant Tornadoes 1880–1989 (Grazulis 1993) and Storm Data (National
Climatic Data Center 1960–99). All thirteen storms were examined in the
same manner. Rawinsonde data at 500hPa, 700 hPa and 850 hPa for the period
commencing 36 hours prior to landfall through 36 hours following landfall
was examined. Analysis involved a smoothed plot of relative humidity at
those levels for evidence of a mid-level dry intrusion. This was accomplished
using Digital Atmosphere software (Vasquez 2000) applying a standard Barnes
analysis scheme utilizing a 2-pass filter. Skew-T/Log-P diagrams were created
using RAOB for Windows (Skewchuk 2002).

TABLE 1.Tropical cyclones
since 1960 producing outbreaks of 20 or more tornadoes with landfall on
the Atlantic or Gulf coast (including the northern Gulf coast of Mexico).

Storm (Year)

Number of tornadoes

Tornadoes with F2 damage

Tornadoes with F3 or greater
damage

Deaths

Injuries

Agnes (1972)

30

7

2

0

84

Alicia (1983)

22

1

0

0

0

Allen (1980)

35

10

1

0

31

Andrew (1992)

56

1

1

2

49

Beryl (1994)

35

8

3

0

58

Beulah (1967)

117

5

6

5

41

Carla (1961)

20

6

7*

14*

335*

Danny (1985)

42

9

5

1

63

David (1979)

34

13

2

2

31

Georges (1998)

42

1

0

0

36

Gilbert (1988)

41

3

0

1

10

Josephine (1996)

30

2

0

0

1

Opal (1995)

32

2

0

1

6

Totals

536

68

27

26

745

*Includes one tornado that produced F4
damage at Galveston as a large waterspout moved onshore; most deaths and
injuries were associated with that event.

3.1 Hurricane Agnes (1972): Hurricane
Agnes moved northward across the eastern Gulf of Mexico and made initial
landfall over the Florida panhandle on 19 June as a Category 1 storm4.
The remnants moved northeastward and back over the Atlantic on 21 June,
after which the storm made a second landfall on 22 June near New York City.

Prior to the
initial landfall, Agnes produced many tornadoes as the right front quadrant
of the storm moved northward over the Florida peninsula. Reports contemporaneous
with the event attributed 15 tornadoes to Agnes but recent reanalysis of
the historical data (applying current Storm Data standards) indicates more
than 20 tornadoes occurred in Florida (Hagemeyer and Spratt 2002). No additional
tornadoes were documented with the second Agnes landfall in southeastern
New York.

At least 28
tornadoes occurred as Agnes tracked northward off the west coast of Florida
on 18 June. Following six pre-dawn tornadoes in the Florida Keys, there
was a lull until the afternoon, then 19 tornadoes occurred between 18UTC
and 03 UTC over the central peninsula, suggesting a strong diurnal influence.
A pronounced relative humidity gradient is seen with dry air to the north
and moist air to the south in an analysis of 500 hPa relative humidity
data at 12 UTC on the morning of 18 June within the outbreak area (Fig.
1). During the ensuing twelve hours, the midtroposphere moistened considerably
as the strong southerly flow advected moist air northward.

This event
can be described as an extended-landfall situation in that the track of
the storm produced an extended period in which conditions favored tornadogenesis
(in this case, across the central Florida peninsula) as the favored quadrant
remained over land for an extended period without the storm center making
landfall. The 00 UTC upper air data from both Tampa and Miami (not shown)
exhibited sufficient storm-relative helicity (hereafter, SRH) for rotating
updrafts, with 0-3 km values at both Tampa and Miami exceeding 280 m2
s-2 (Droegemeier et al. 1993). It is unlikely that the dry air
ahead of Agnes is responsible for all of the tornadoes, but the occurrence
of daytime tornadoes in the gradient between dry air and moist air will
be seen in the ten additional qualifying cases reviewed below. Based
on the evidence at hand, this appears to be a case where the outbreak can
be sufficiently attributed to a mid-level dry intrusion to justify inclusion
of this case.

4All
references to storm category are based on Saffir and Simpson (1974).

3.2 Hurricane Allen (1980): Hurricane Allen
passed near the Yucatan peninsula on 7 August and made landfall on the
lower Texas coast near Brownsville about 06 UTC on 10 August, then continued
moving to the west-northwest. Allen was a Category 5 storm 400 km east-southeast
of Brownsville but had weakened to Category 3 at the time of landfall.

The storm
produced numerous tornadoes across south central Texas on 10 August.
Stiegler and Fujita (1982) surveyed the San Marcos, Texas tornado (one
of several in the Austin-San Marcos area) and found a 12.5 km long damage
path with evidence of multiple vortices and some F3 damage5.
Another strong tornado (damage rated F2) caused $50 million in damage at
the Austin Municipal Airport about the same time. At least 16 of Allen’s
29 tornadoes occurred in central Texas, in both the right-front quadrant
and the northeast quadrant, some 250 to 400 km from the storm center.

Examination
of the sounding data revealed a pronounced dry intrusion that appears to
have been entrained into Allen’s circulation from the east and southeast.
Best depicted at the 700 hPa level (Fig.
2), the intrusion rotated westward over a period of 24 to 36 hours,
producing a very sharp north-south gradient of relative humidity above
the Austin-San Antonio area during the day on 10 August. The intense mid-level
gradient coincides with the outbreak area.

5 All
F-scale references are based on Fujita, 1981.

3.3 Hurricane Andrew (1992): Hurricane
Andrew made landfall twice. On 24 August, the storm crossed southern Florida
as a Category 5 storm (Landsea et al., 2003, manuscript submitted to Bull.
Amer. Meteor. Soc.), then turned northwest and made a
second landfall as a Category 1 storm on the Louisiana coast before sunrise
on 26 August. The storm turned to the north and then northeast and passed
near Tupelo, MS around 17 UTC on the 27 August.

Andrew may
have been the most prolific producer of tornadoes since Hurricane Beulah
(1967). All of the documented tornadoes occurred with the second landfall,
when numerous tornadoes occurred across Louisiana, Mississippi and Alabama
from the evening of 25 August through the afternoon of 27 August. The most
damaging tornado occurred prior to the second landfall when one moved to
the west-northwest across an area just west of New Orleans, LA on the evening
of 25 August. This tornado produced F3 damage along a path 14 km in length,
and caused two deaths and 32 injuries. A rawinsonde observation made within
three hours of the event at nearby Slidell, LA revealed an environment
with CAPE around 700 J kg-1 and 0-3 km SRH in excess of 300
m2 s-2.

Most of the
tornadoes occurred on 26 August: nine between 12 UTC and 18 UTC and thirteen
from 18 UTC to 00 UTC. Ten more were reported in the next three hours.
The sounding data from 12 UTC on 26 August revealed evidence of an extensive
area of dry air north and east of the storm center. The Centreville, AL
sounding (not shown) found a dew point depression of 20° C at 700 hPa
and the profile was quite dry above ~850 hPa. The extent of the dry air
intrusion is best depicted at 700 hPa (Fig.
3).

Close to the
area of tornadogenesis, the vertical extent of the dry intrusion was sampled
by the 12 UTC sounding from Jackson, MS, which found relative humidity
averaging less than 65 percent through most of the layer between 850 hPa
and 500 hPa, with several points below 50 percent around the 725 hPa level.
A comparative plot6
of both the 12 UTC and 00 UTC soundings (Fig.
4) shows that the atmospheric column moistened during the day, but
without much change in the low-level lapse rate.

By the 00
UTC, the obvious signs of the dry intrusion at mandatory levels had been
reduced, perhaps by the strong, deep southerly flow in the eastern semi-circle
of the storm. However, 00 UTC sounding at Centreville, AL (not shown) revealed
that relatively dry air persisted in the layer between 800 hPa and 600
hPa where the average relative humidity was around 60%. Dry air persisted
at mid and upper levels over Georgia and northern Florida, where mid-level
dew point depressions exceeded 20° C.

6All
comparative sounding plots have been truncated at 400 hPa for better resolution
of the lower and middle levels.

3.4 Tropical Storm Beryl (1994): Beryl developed
close to the central Gulf coast southeast of Mobile, AL on 14 August and
moved northeast, making landfall near Panama City, FL late on the afternoon
of 15 August. Although no tornadoes accompanied the landfall, Beryl’s
remnants moved northward into Georgia and an outbreak of 28 tornadoes occurred
on 16 August, mainly in South Carolina.

This case
was examined by Vescio et al. (1996), who found that dry air in a deep
layer above the surface was entrained into the northeast quadrant of the
system over portions of the Carolinas. The dry intrusion resulted in steeper
lapse rates and greater instability. A comparison of the 00 UTC and the
12 UTC soundings on 16 August at Charleston, SC (Fig.
5) reflected a drastic change. Although the winds had backed through
a deep layer, providing more of an onshore flow, significant drying had
occurred through a deep layer from just above the surface to near 500 hPa.
The sounding profile at 00 UTC does reflect a substantial increase in the
low-level lapse rate through approximately the lowest 200 hPa of the sounding.

The tornadoes
reflected a strong diurnal influence, as all but two occurred between 17
UTC and 00 UTC. Several of the tornadoes had long tracks (>15 km) and two
had path lengths exceeding 40 km. Two of the tornadoes produced damage
rated F3 and eight produced damage rated F2. The outbreak was centered
in the sharpest portion of the relative humidity gradient at 700 hPa (Fig.
6), centered near Columbia, SC.

3.5 Hurricane Beulah (1967): Hurricane
Beulah crossed the Yucatan peninsula on the 17 September and was briefly
rated a Category 5 storm over the Gulf of Mexico prior to landfall. However,
Beulah weakened to Category 3 just prior to landfall on the morning of
20 September, which occurred over Padre Island northeast of Brownsville,
TX. Beulah continued toward the north-northwest for 18 hours, then
turned toward the west, and finally toward the southwest, moving into northern
Mexico.

Beulah’s tornado
productivity was highest on the day of landfall (20 September), but the
storm produced at least 20 tornadoes on 21 September (after the westward
turn) and an additional 21 tornadoes on the 22 September (while moving
to the southwest across the Rio Grande and into northern Mexico). Examination
of the sounding data reveals a dry intrusion, again best seen at 700 hPa,
which appears to be entrained into the storm’s circulation from the east.
The analysis reveals a significant gradient of relative humidity analyzed
across the upper Texas coast and into central Texas on the morning of the
20 September. This gradient remained in place through the subsequent 48
hours, gradually shifting to the southwest. The dry layer became increasingly
apparent on the Lake Charles, LA soundings at 00 UTC on 19 September and
12 UTC on 20 September (neither shown).

The dry layer
became faintly evident on the Victoria, TX sounding 00 UTC on 21 September
and was pronounced by 12 UTC on 22 September, as seen in a comparison of
those soundings (Fig. 7). The diagram depicts
a significant change in the environmental lapse rate commencing at about
the 850 hPa level and extending upward. The change coincides with a large
drop in relative humidity that commences at 900 hPa and extends upward
through most of the sounding. Surface-based CAPE (SBCAPE) had increased
to almost 2000 J kg-1 at that time.

An analysis
of 700 hPa relative humidity at both 12 UTC and 00 UTC (not shown) on 20
September revealed a significant dry to moist (east to west) gradient that
coincided with the area of tornadogenesis. Tornadoes began occurring well
before sunrise in the coastal plains between Victoria and Houston as the
right-front (northeast quadrant) moved onshore, contemporaneous with the
center’s landfall north of Brownsville.

The exact
number of tornadoes on 20 September is a matter of some conjecture because
the recorded locations and times for the events occasionally seem to overlap.
The State Climatologist reported a total of 115 tornadoes for Beulah, and
67 tornadoes on 20 September, both totals that were reached by reviewing
"all storm reports collected by ... Weather Bureau Offices in the areas
affected ..., by examining 156 Hurricane Report questionnaires (prepared)
by persons in the affected area, and by scanning more than 700 newspaper
clippings pertaining to ... descriptive evidence of tornado occurrence"
(Orton 1970).

Tornadogenesis
on 21 September and 22 September displayed a distinctly diurnal pattern,
with most of the tornadoes occurring during daylight hours. Analysis of
700 hPa relative humidity at 00 UTC on 21 September (Fig.
8) found a significant relative humidity gradient across south central
and southeastern Texas, over the area in which the outbreak occurred on
21 September. At 12 UTC on 22 September, the gradient was centered near
Alice, west of Corpus Christi (not shown). All of the tornadoes on 21 and
22 September occurred in the "rear" quadrants of the storm relative to
forward motion, but in the northeast quadrant of the storm relative to
the storm’s location.

3.6 Hurricane Danny (1985): Hurricane
Danny made landfall as a Category 1 storm on the southwest Louisiana coast
around 17 UTC on the 15 August. The storm continued northward, passing
near Monroe, LA around 12 UTC on 16 August, then turned more to the northeast,
passing near Muscle Shoals, AL around 06 UTC on 17 August. Danny produced
numerous tornadoes on 16 August, the day after landfall. A strong diurnal
trend was noted, as most of the tornadoes (20) occurred in a six-hour period
between 15 UTC and 21 UTC across portions of northern Alabama and southern
Tennessee.

McCaul (1987)
provided a detailed analysis of this tornado outbreak. He noted that some
of the tornadoes occurred in “tornado families”, produced relatively long
damage tracks, and in some instances, produced F3 damage. He concluded
that one of the factors in the outbreak may have been a mid-and upper-level
dry intrusion, noting that satellite imagery depicted a narrow but well-defined
mid- and upper-level dry intrusion in western Alabama that appeared to
be continuously connected to a large source of mid-level dry air over Mississippi,
Louisiana and Arkansas. However, analysis of relative humidity at 500 hPa
at 12 UTC (Fig. 9) suggests that dry air was
already very close to, but just south of, the outbreak area.

This source
may have aided in initiation of the outbreak with the dry intrusion noted
by McCaul aiding in its continuation. Significant drying through a substantial
layer of the midtroposphere (800 to 500 hPa) is seen in the comparative
plot of the Jackson, MS soundings at 12 UTC and 00 UTC on 16 August (Fig.
10). The evening (00 UTC) sounding revealed a significant increase
in the low-level lapse rate (surface to 950 hPa) but this was in the westerly
flow in Danny's wake.

3.7 Hurricane David (1979): Hurricane
David passed south of Puerto Rico and struck the island of Hispaniola as
a Category 5 hurricane on 31 August. Weakened by interaction with the island,
the storm continued northwest and then north, never regaining its prior
intensity.

David came
very close to making continental landfall twice. On 3 and 4 September,
the storm was just offshore from the east coast of Florida moving northward.
Ten brief, mostly weak tornadoes occurred along the immediate coastal area.
The storm continued northward to a decisive landfall near Charleston, SC
on the night of 4 September as a strong Category 1 storm. Four additional
brief, weak tornadoes occurred near and north of Myrtle Beach coincident
with landfall. There is no evidence of a dry intrusion with any of these
tornadoes.

David continued
northward and produced an outbreak of 19 tornadoes in the District of Columbia,
Maryland, Delaware and Pennsylvania on 5 September. The tornadoes occurred
in a strong gradient of relative humidity at 500 hPa between mid-level
dry air to the south and moist air to the north (Fig.
11). These tornadoes displayed a strong diurnal trend, with 14 of the
19 tornadoes occurring between 18 UTC and 01 UTC. Several tornadoes had
remarkably long tracks (>25 km) and were quite intense (two produced F3
and eight produced F2 damage). A comparison plot of soundings for Washington,
DC at 12 UTC and 00 UTC on 5 September (Fig.
12) shows drying through the lower troposphere despite significant
moistening in the layers above 700 hPa. SBCAPE at Washington was approximately
900 J kg-1 on both the 12 UTC and 00 UTC soundings. Perhaps
of greater importance, the SRH at Washington was very high during the outbreak
(from ~280 m2 s-2 in the 0-2 km layer7
to more than 500 m2 s-2 in the 0-3 km layer). There
was only a minor increase in the low-level lapse rate between the 12 UTC
and 00 UTC soundings.

7Johns
et al. (1993) examined the relationship between CAPE and 0-2 km helicity
for tornadoes rated strong and violent. In four TC cases involving strong
tornadoes, the authors found relatively low values of both CAPE and helicity.

3.8 Hurricane Georges (1998): Hurricane
Georges crossed Puerto Rico, Hispaniola and Cuba. The storm attained Category
4 status before reaching Puerto Rico, but was weakened by interaction with
the islands. Georges departed Cuba heading northwest, then turned northward
across the central Gulf of Mexico to a continental landfall shortly after
06 UTC on 28 September just west of Gulfport, MS. A Category 1 storm at
landfall, Georges moved a short distance inland, then slowed and turned
abruptly eastward, moving parallel to the Gulf coast, a motion that permitted
Georges to maintain tropical storm status for an extended period.

The unusual
motion of Georges following landfall resulted in an extended landfall event
as the storm remained within ~100 km of the coast as it moved slowly eastward,
producing an extended period of strong onshore flow from the very warm
waters of the Gulf of Mexico. Twenty tornadoes occurred on 28 September,
mainly over southeastern Alabama.

The 12 UTC
analysis for 28 September at 700 hPa (Fig.
13) and 500 hPa (not shown) demonstrated a significant relative humidity
gradient across the area where the tornadoes subsequently occurred, with
dew point depressions of 8° to 10° C at 700 hPa in the zone from
Tampa to Tallahassee to Atlanta. The timing of the outbreak (afternoon
and evening) on the 28 September suggests the involvement of diurnal influences.

The Tallahassee
sounding at 00 UTC on 28 September) (not shown) found a distinct dry intrusion
in the layer between 650 hPa and 450 hPa, where the average relative humidity
was less than 35 percent. Twelve hours later, the 700 hPa relative humidity
analysis for 12 UTC on 29 September (Fig. 14)
found the area of very dry air was still present over northwestern Florida
and adjacent areas of Alabama and Georgia. The dew point depression at
700 hPa had increased to 6° C at Tallahassee, 22° C at Tampa and
17° C at Jacksonville. At 500 hPa (not shown), the dew point depression
at Tallahassee (21?C) continued to reflect a very dry pocket of air. The
remnants of Georges produced 17 more tornadoes on the 29 September over
southeastern Alabama, southern Georgia and northwestern Florida, with slightly
more than two-thirds of the tornadoes occurring during the afternoon and
early evening hours.

3.9 Hurricane Gilbert (1988):
Hurricane Gilbert crossed Yucatan as a Category 5 storm on 14 September
and made landfall as a Category 4 storm on the northeast coast of Mexico
about 210 km south of Brownsville, TX on 16 September.

Gilbert, a
very large hurricane at the time of landfall, produced 12 tornadoes in
the lower Rio Grande Valley of Texas during and immediately after landfall.
The larger outbreak (28 tornadoes) occurred the day after landfall on 17
September. These tornadoes occurred at distances of 375 to 750 km from
the storm center, which was then about 350 km inland and moving over the
increasingly mountainous highlands south of Monterrey, Nuevo Leon. Examination
of the sounding data from 17 September revealed a similar pattern to that
seen in the other cases but the dry intrusion is slightly better depicted
at the 500 hPa level than at 700 hPa. The 12 UTC analysis at 500 hPa on
17 September (not shown) found a pronounced relative humidity gradient
extending inland toward central Texas from the upper Texas coast from a
relative humidity minimum just south of Lake Charles. Twelve hours later,
the 500 hPa analysis (Fig. 15) found that
the gradient had intensified and advected westward. Analysis of the 00
UTC sounding at Stephenville, TX (not shown) yielded SBCAPE of almost 1400
J kg-1 and sufficient low-level support for updraft rotation
(SRH of 150 m2 s-2 in the 0-1km layer and 210 m2
s-2 in the 0-2 km layer).

An interesting
subset of Gilbert's tornadoes occurred at a remarkably extreme range (~750
km) from the remnants of the storm center. Ten tornadoes occurred in the
counties just east of San Angelo, TX along a baroclinic surface boundary
that appears to have developed in place as an indirect result of the mid-level
dry intrusion to the east. The outer rain shield of Gilbert cooled the
lower troposphere overnight on the 16th/17th as it rotated east to west
across Texas. Then significant insolation in the areas affected by the
dry intrusion warmed the lower troposphere during the morning and early
afternoon of the 17th. The result was a boundary oriented northeast-southwest
with cool, moist air to the west and warm, moist air to the east (Fig.
16). All of the tornadoes occurred between 17 UTC and 22 UTC close
to this low-level boundary. As noted in Sec. 2.2, low-level meso-beta scale
baroclinic boundaries have been implicated in tornadogenesis in both TC
and mid-latitude supercells (Pietrycha and Hannon 2002; Edwards et al.
2000; Markowski et al. 1998).

3.10 Tropical Storm Josephine
(1996): Josephine
developed in the Gulf of Mexico on 4 October and moved to the northeast
along a stalled baroclinic zone. The storm made landfall on the evening
of 7 October at Apalachee Bay (FL) shortly before 06 UTC. Never a
very robust TC, the system quickly lost its tropical characteristics as
it moved rapidly northeastward into Georgia on 8 October and continued
moving to the northeast.

Prior to landfall,
Josephine spawned an outbreak of 18 tornadoes over the central Florida
peninsula on 7 October. At 12 UTC, the sounding profiles at both Tampa
and Tallahassee were saturated from the surface to well above 500 hPa;
at Miami, the profile hinted at the beginning of a dry intrusion above
700 hPa; and at Key West, there was evidence of significant drying from
850 hPa upward. The flow at Key West was nearly unidirectional from the
south-southwest through almost 500 hPa. Twelve hours later, evidence of
a mid-level dry intrusion was profound at both Tampa and Miami. The dry
intrusion is best depicted by the analysis at 500 hPa (Fig.
17). The tornadoes over the Florida peninsula presented a distinct
diurnal pattern, as fifteen of the eighteen occurred between 15 UTC and
00 UTC, all prior to the actual landfall.

At 00 UTC,
the Tampa surface wind was from 215° at 7.5 m s-1, but very strong
winds (~30 m s-1) were detected just above the surface. Not
far inland from Tampa, surface winds were backed, with the flow from approximately
155° at around 7 m s-1. Most of the tornadoes occurred between
18 UTC and 00UTC in the area where the backed flow was located. Adjustment
of the 00 UTC Tampa wind profile to a surface flow from 155° at 7 m
s-1 yielded a very potent SRH value of 268 m2 s-2
in the 0-1 km layer and 385 m2 s-2 in the 0-2 km
layer.

As the remnants
moved into the Carolinas, a brief flurry of eight tornadoes occurred along
the coast on the morning of the 8 October. Hudgins and Frederick (1997)
examined some of the 8 October tornadoes and found that they occurred along
a strong coastal front located just inland of the coast. Objective analysis
of surface data at 12 UTC confirms their analysis (Fig.
18).

The tornadoes
were relatively weak and brief, although one did produce F2 damage. All
of the tornadoes occurred between 0930 UTC and 1400 UTC, just ahead of
the surface cyclone, in an area where winds of 14 to 23 m s-1impinged
on the warm front. The Newport, NC (KMHX) 12 UTC rawinsonde reflected 0-2km
SRH exceeding 600 m2 s-2. These tornadoes, produced
by the remnants of a TC about to leave the continental landmass and move
back over the ocean, are consistent with the pattern described by Edwards
(1998). Hudgins and Frederick (1997) mention that mid-level drying is shown
above the 700 hPa level.

The tornadoes
occurred prior to 15 UTC, so diurnal influences did not play a role, but
the mid-level dry air could have contributed to this part of the outbreak
through other mechanisms, such an entrainment into mini-supercells and
downward transport to the near-surface environment, as discussed hereafter
in Sec. 5.2. As was the case with Agnes, it is unlikely that the dry intrusion
was responsible for all of the tornadoes, but this appears to be another
case where the outbreak can be sufficiently attributed to a mid-level dry
intrusion to justify inclusion of this case.

3.11 Hurricane Opal (1995): Hurricane
Opal developed over the Yucatan and became a tropical storm on 30 September
as it moved into the northern Bay of Campeche. Opal became a hurricane
on 2 October and began a north-northeast motion at increasing forward speed.
Landfall was near Pensacola, FL on the evening of 4 October. Opal will
be remembered not only for its tornado outbreak but also for its sudden
fluctuation in intensity (Shay et al. 2000). Between late evening on 3
September and sunrise on 4 September, the storm rapidly intensified to
a Category 4 hurricane, but then just as quickly weakened again to Category
1 prior to landfall. The remnants of Opal moved rapidly northeast across
Alabama, Tennessee and Kentucky on 5 October.

Opal produced
numerous tornadoes on 4 October in a twelve-hour period centered around
landfall. Inspection of the locations and times of tornado occurrence suggests
two episodes of tornado activity: between 15 UTC and 19 UTC, and again
between 22 UTC and 02 UTC. The initial burst of tornadoes (9) can be classified
as an outer rainband event. The subsequent burst of tornadoes (13) was
related to landfall of the core of the storm. In both events, the tornadoes
occurred within 120 km of the coast.

Visible satellite
imagery from the morning of 4 October depicted a clear slot (a zone of
cloud erosion) in the eastern semi-circle of Opal’s circulation. The GOES-8
visible image from 1602 UTC (Fig. 19) shows
that the clear slot extended to a point very close to Pensacola, FL, and
a zone of intense convection was located just on the north and northeast
side of this area. The initial outbreak of tornadoes was underway within
the band of intense convection coincident with this image.

Analysis of
the 12 UTC sounding data at both 700 hPa and 500 hPa, as well as water
vapor satellite imagery (not shown) suggests that the dry intrusion occurred
in a narrow channel that was west of both Tampa and Tallahassee, perhaps
coinciding with the clear slot feature seen in the satellite image.
Analysis of relative humidity at 500 hPa at 00 UTC (Fig.
20) indicated that the drying had spread eastward to Tallahassee, FL.
Although drying was present between 700 hPa and 500 hPa in both soundings,
the full extent of the dry intrusion was best detected in the evening sounding
from Tallahassee (not shown), where the dew point depression at 500 hPa
increased by 15° C from the 12 UTC sounding.

The dry intrusion
in the eastern semicircle appears to have originated well to the northwest
of Opal several days prior to the landfall. Rodgers et al. (1998) found
that the dry region intruded cyclonically around the western and southern
regions of Opal. They attributed the origin of the dry air to vertical
motions within a mid-latitude trough that crossed the southern plains,
eventually producing Opal’s northward acceleration and concluded that dry
air intruded quite close to Opal’s center by 12 UTC on 4 October.

4. DISCUSSION OF NON-QUALIFYING
CASES

Two of the
thirteen outbreak cases do not appear to have been related to mid-level
dry intrusions. It is worthy of note that although these events appear
to be contrary to the pattern seen in the eleven qualifying cases, they
did produce a number of significant tornadoes.

4.1 Hurricane Alicia (1983) Hurricane
Alicia made landfall on the upper Texas coast near Galveston before sunrise
on 18 August as a Category 3 storm, and then continued toward the north-northwest.
Examination of the location and times of the tornadoes attributed to Alicia
reveals that virtually all were associated with the core of the storm as
opposed to the outer rainbands. Most occurred in the immediate area of
landfall, in the greater Houston-Galveston area. Inspection of the pertinent
upper air data revealed no evidence of a dry intrusion at either 700 hPa
or 500 hPa, although at 500 hPa very dry air existed in all quadrants beyond
~350 km. Examination of satellite imagery from the morning of landfall
(although primitive compared to today’s imagery) does not suggest evidence
of a dry intrusion into the storm’s main circulation. Coupled with the
fact that most of the tornadoes were associated with the core of the storm,
Alicia was determined to be non-qualifying.

***

4.2 Hurricane Carla (1961) Hurricane
Carla made landfall on the middle Texas coast on the evening of 11 September.
Rated a Category 5 storm over the western Gulf of Mexico, Carla had weakened
to a Category 3 storm at landfall. The storm continued northwest into central
Texas on 12 September. Tornado production began on the 10th when the storm
was 275 to 300 km south of Galveston. All of the tornadoes on 10 September
and all but three on 11 September occurred in Louisiana. Relative to storm
motion at the time, all but one of the Louisiana tornadoes would be classified
as having occurred in the right-rear quadrant. However, relative to true
north, they were in the northeast quadrant of the storm.

But for the
Louisiana tornadoes, Carla would not qualify for inclusion in this study.
All of these tornadoes (on 10 and 11 September) occurred at rather extreme
range from the storm (400 to 700 km) and well prior to landfall. Examination
of mid-level relative humidity reveals little evidence of a dry intrusion,
and these tornadoes (antecedent to landfall by 18 to 36 hours) suggest
the operation of factors other than a mid-level dry intrusion involving
outer rainbands. The subsequent tornadoes in Texas on 11 September and
the tornadoes in both states on the 12 September were associated either
with the core of the storm or with outer rainbands, but the number of these
tornado falls well below the "outbreak" threshold.8

***

8Included
in the 12 September events is the early morning (0915 UTC) tornado that
struck Galveston Island after initially developing offshore as an intense
waterspout (Grazulis 1993). This tornado caused damage rated F4 and
according to Grazulis, may have been the most intense tornado ever to strike
Galveston. This event stands for the proposition that TC tornadoes need
not be related to a dry intrusion to produce very significant damage.5. DISCUSSION: THE ROLE OF MID-LEVEL
DRY INTRUSIONS

As suggested
in Sec. 2.3, there are a number of issues involved in tornado outbreaks
associated with landfalling TCs and the involvement of mid-level dry intrusions.
Various explanations have been offered, with the most frequent being an
increase in convective instability due to an increase in the environmental
lapse rate, other alterations of the thermodynamic structure of the tropical
cyclone environment with substantial enhancement of CAPE and surface-based
instability, and enhanced evaporative cooling within the rear-flank downdraft
(RFD) with modulation of the development of mesocyclones in the outer rainband
as a result.

***

5.1 Sources of the Mid-Level
Dry Air: McCaul (1987)
raised a crucial issue, noting that if the enhancement of convective instability
due to dry intrusions aloft is indeed an important factor in triggering
large outbreaks of TC tornadoes, it is important to discern the critical
factors that dictate which hurricanes will ingest dry air in the most destabilizing
manner. Based on the storms examined herein, it appears that tornado outbreaks
are more likely when the mid-level dry intrusion produces a relatively
sharp gradient of relative humidity in the storm quadrant where parameters
support the development of mesocyclones. However, the source of the dry
air that was ingested by the storms was found to vary.

To examine
this issue, 6-hourly data from the Climate Diagnostics Center's NCEP/NCAR
Reanalysis Project (Kalnay et al. 1996) was used to examine relative humidity
distribution in the mid-levels for an extended period and over a broad
area prior to storm landfall. Animated loops of these graphics suggest
two primary patterns. One pattern (arbitrarily designated Type 1) involved
a mass of dry air that impinged on much of the northern or northwestern
semi-circle of the storm's outer circulation. In these cases, the dry air
appeared to gradually divide into two lobes as the storm advanced, with
one to the northwest and the other to the northeast of the storm center.
In the other pattern (arbitrarily designated Type 2), the dry air was ingested
into the storms from a reservoir of mid-level dry air, most often (but
not exclusively) located in the eastern semi-circle of the storm.

Agnes, Beulah,
David and Gilbert were found to be Type 1 storms. Allen, Andrew, Beryl,
Danny, Georges, and Josephine were found to be Type 2 storms. Opal was
found to exhibit both patterns. The time-series loop suggested that Opal's
envelope of tropical air gradually displaced the pre-existing dry air over
the southeastern U.S., but that the more significant dry intrusion originated
with the air that wrapped into the circulation in the strong cyclonic flow
from the south and southwest.

***

5.2 Convective Instability and
the Diurnal Signal: The qualifying
cases have a strong diurnal signal detected in the temporal distribution
of the tornadoes. Almost two-thirds (65%) of the tornadoes in the eleven
qualifying cases occurred between 15 and 24 UTC (daylight hours in the
region examined). By contrast, in the two cases excluded because of the
absence of evidence of a dry intrusion, only 37% of the tornadoes occurred
in the same period. Some of the previously published research has
addressed this issue. McCaul (1991) found that the tornadoes in his proximity
database featured a "distinct maximum in early and mid afternoon", with
54% of the tornadoes occurring between 9am and 6pm local solar time.
He concluded that "this finding suggests that solar radiational heating
might be important even in the mostly cloudy environments characteristic
of landfalling hurricanes." Novlan and Gray (1974) noted a "weak frequency
peak around 1100LST" but did not attribute any significance to this finding.
Hill et al. (1966) noted that other researchers had examined and rejected
the notion of a temporal distribution because they were examining only
one or two storms. They found a general preference for the period from
local noon to local midnight, but declined to attribute appreciable significance
to the finding. Orton (1970) found a significant peak for Beulah’s tornadoes
between 9am and 6pm local time.9

The manner
in which the mid-level dry intrusion enhances tornadogenesis has been the
subject of some research. McCaul (1991) found that overall there was a
negative correlation between CAPE and tornado potential on average for
all of his cases. He noted that CAPE was typically at a minimum near and
to the left of the center’s track, and maximized well to the right of the
center’s track. He found that in the vertical, most of the tropical cyclone’s
CAPE is found a levels below 650 hPa, because of warming above that level.
As noted previously, Vescio et al. (1996) suggested that mid-level dry
intrusions have the potential to substantially alter the thermodynamic
structure of the tropical cyclone environment (with substantial enhancement
of CAPE and surface-based instability).

Proximity
soundings (defined in McCaul {1991} as those occurring within 3 h and 185
km of a tornado event) for the closest one or two stations in the outbreak
cases herein were examined for evidence that mid-level dry intrusions might
produce a consistent pattern with respect to temperature lapse rates. Despite
the use of a variety of levels (950-700 hPa, 850-500 hPa, 700-500 hPa),
no consistent pattern was discernible. In each case, the morning and evening
soundings on the outbreak day were examined. On average, the lapse rates
between in the 950-700 hPa layer increased by less than 0.2° C, the
lapse rates in the 850-500 hPa layer were unchanged, and the lapse rates
in the 700-500 hPa layer decreased by slightly more than 0.2° C.
This finding may reflect that the sounding sites were too far removed from
the tornadic cells to detect the alteration of the near-storm environment
by the mid-level drying, or it may suggest that other factors were involved.

As noted previously
in Sec. 2.3, Cione et al. (2000) examined changes in air temperature and
sea surface temperature changes in 37 hurricanes between 1975 and 1998,
and concluded that the change in the spread between sea surface temperature
and TA was primarily due to a reduction in TA (and not the result of adiabatic
expansion), and proposed that one explanation “may be that outside the
hurricane inner core, unsaturated convective downdrafts act to dry and
evaporatively cool the near-surface environment”. They also reviewed previous
research indicating that convective downdrafts are capable of bringing
cooler air to the surface in tropical systems, provided the air aloft is
sufficiently dry.

Spratt et
al. (2000) documented a convective downdraft in an outer rainband cell
of Hurricane Bonnie (1998) utilizing data from a dropsonde launched from
a
NOAA Hurricane Research Division (HRD) aircraft as the hurricane neared
the North Carolina coast. The dropsonde fortuitously advected into a mini-supercell
soon after its release, and remained entrained in the cell until just prior
to reaching the surface. The dropsonde documented a steep lapse rate during
descent, indicative of a layer of surface-based instability. Vertical velocity
data from the aircraft was retrieved and it confirmed downward motion associated
with the low-level theta-e minima, thus confirming the origin of the parcel
(aloft, descending within downdraft core)10.

The transport
of mid-level dry air to or near the surface in TCs does not directly explain
the diurnal signal seen in the outbreak cases. Another possible mechanism
involves the erosion of mid and upper tropospheric cloudiness by the intruding
dry air, allowing full insolation to penetrate to near (if not all the
way to) the surface, thereby enhancing convective instability on a relatively
local scale. But ultimately, the answer to how mid-level dry intrusions
are related to tornadogenesis in landfalling TCs may depend upon model
simulations.

One such effort
has produced interesting (if preliminary) results. Romine and Wilhelmson
(2002) have conducted numerical model simulations of the Hurricane Opal
(1995) tornado outbreak, one of the qualifying cases herein. They have
successfully simulated the dry intrusion in the storm’s eastern semi-circle,
and are presently using the NCOMMAS model, a derivative11
of the Collaborative Model for Multiscale Atmospheric Simulation (COMMAS)
(Gilmore and Wicker 1998) in simulations focusing on the role that the
dry air may play with respect to the evolution of mini-supercells in the
area affected by the dry intrusion.

***

9Note that
seven of the thirteen cases examined herein were part of McCaul’s dataset;
the other six occurred subsequent to the last case included in his
dataset.10Spratt
2000, personal communication.11Romine
2003, personal communication.6. DISCUSSION: NULL CASES

Another significant
issue with respect to the primary findings discussed in Sec. 3 is whether
there have been landfall events with detectable mid-level dry intrusions
that did not produce tornado outbreaks as defined herein. To examine this
issue, the 4-times daily data from the Climate Diagnostics Center's NCEP/NCAR
Reanalysis Project was used to examine relative humidity in the mid-levels
for all TCs making landfall in the study area during the specified period.
All landfalling cyclones were examined in the same manner: soundings were
examined at 500 hPa, 700 hPa and 850 hPa for the period commencing 36 hours
prior to landfall through 36 hours following landfall. There were three
events with definitive evidence of a mid-level dry intrusion in which the
number of tornadoes failed to reach at least twenty (the minimum established
for inclusion as “outbreak cases” herein), but each of those cases did
involve a significant number of tornadoes.

Hurricane
Earl (1998) made landfall around 06 UTC on 3 September near Panama City,
FL. The remnants of the storm moved northeast to near Wilmington, NC at
06 UTC on 4 September. Edwards et al. (2000) described the tornado production
by Earl as bimodal because the tornadoes occurred in two phases: on the
day prior to landfall and on the following day as the remnant circulation
was about to move back over the open ocean. A total of 15 tornadoes occurred,
about half in the pre-landfall stage and the others in the “exit” phase
(Edwards, 1998). Examination of the sounding data for 00 UTC on 03 September
reveals a pronounced relative humidity gradient at 700 hPa overlying the
area of the pre-landfall tornadoes, with drier air to the southwest of
Tampa and moist air to the northeast. The mid-level dry air continued advecting
to the northeast and produced a pronounced relative humidity gradient at
700 hPa over the area of the “exit” phase tornadoes at 00 UTC on 4 September.

Hurricane
Edith (1971) made landfall on the Louisiana coast on 16 September.
Edith continued to the northeast through Louisiana and Mississippi and
produced sixteen tornadoes on the day of landfall. A strong diurnal trend
was noted, with most of the tornadoes (11) in a nine-hour period between
15 UTC and 00 UTC and with peak occurrence shortly after 18 UTC. Examination
of the sounding data revealed a pronounced dry intrusion that appears to
have been entrained into the Edith's circulation from the east and southeast
(Type 2). Best defined at the 700 hPa level, the intrusion remained east
of the center of the storm following landfall. This non-qualifying outbreak
of tornadoes occurred directly beneath the intense relative humidity gradient.

Hurricane
Floyd (1999) made landfall before sunrise on 16 September near Wilmington,
NC. Floyd produced eighteen tornadoes across mostly coastal sections of
North Carolina on 15 September (prior to landfall). Although there was
a significant diurnal trend in this outbreak, suggesting the possibility
of a mid-level dry intrusion, it appears that other factors were involved.
Pietrycha and Hannon (2002) found that 16 of the 18 tornadoes developed
immediately along and/or within the warm side of a coastal front that developed
and moved onshore ahead of the storm as it approached the coast. Analysis
of the 00 UTC sounding data (on the evening of 14 September) revealed a
pronounced relative humidity gradient at 500 hPa (less so at 700 hPa) with
very dry air to the northwest and very moist air to the southeast (Type
1). By 12 UTC on 15 September, the intense gradient at 500 hPa remained
but the 700 hPa gradient had relaxed, perhaps due to the arrival of the
moist envelope associated with Floyd.

By 00 UTC
the gradient at 500 hPa was arranged in a more west to east (dry to moist)
configuration and the 700 hPa gradient was beginning to suggest some wrapping
of dry air from the southwest around the south side of Floyd’s circulation.
The intense 500 hPa gradient overlay the zone of tornadogenesis during
the daylight hours on 15 September.

7. DISCUSSION AND CONCLUSIONS

Table 2 contains
a comparison of the three classes of storms (qualifying, non-qualifying
and null) including the number of tornadoes produced by storms with mid-level
dry intrusions that produced outbreaks (qualifying), storms without mid-level
dry intrusions that produced outbreaks (non-qualifying), and storms with
mid-level dry intrusions that failed to produce outbreaks (null) by number
of tornadoes, significant F-scale damage, and casualties.

TABLE 2. Comparison of tornadoes
produced by storms with mid-level dry intrusions that produced outbreaks
(qualifying), storms without mid-level dry intrusions that produced outbreaks
(non-qualifying), and storms with mid-level dry intrusions that failed
to produce outbreaks (null) by number of tornadoes, F-scale damage, and
casualties.

Class

Number of tornadoes

Tornadoes with F2 damage

Tornadoes with F3 or greater
damage

Deaths

Injuries

Qualifying

494

61

20

12

410

(Avg. per storm)

(45)

(6)

(2)

(1)

(41)

Non-qualifying

42

7

8

14*

335*

(Avg. per storm)

(22)

(4)

(3)

(5)*

(168)*

Null

51

10

2

2

33

(Avg. per storm)

(17)

(3)

(1)

(0)

(11)

*This classification (and its averages)
appear somewhat distorted by the single event with unusually high casualty
figures (F4 at Galveston during Hurricane Carla).

In an attempt
to discern significant differences in the environments of storms that had
dry intrusions and produced outbreaks, those that had no dry intrusion
and yet produced outbreaks, and those that had dry intrusions but failed
to produce outbreaks, composite soundings were generated utilizing the
closest one or two stations for which soundings were made in or close to
the favorable right front (northeast) quadrant closest to the time of the
tornado events or the outbreak. The results are presented in Table 3.

TABLE 3. Comparison of average
LCL height and of dew point depression at various levels in storms with
mid-level dry intrusions that produced outbreaks (qualifying), storms without
mid-level dry intrusions that produced outbreaks (non-qualifying), and
storms with mid-level dry intrusions that failed to produce outbreaks (null)
.

Class

LCL (in meters)

Surface (Tdd in °C)

925(Tdd in °C)

850(Tdd in °C)

800(Tdd in °C)

700(Tdd in °C)

600(Tdd in °C)

500(Tdd in °C)

Qualifying

141

1.5

1.8

3.0

3.4

4.4

6.6

7.0

Non-qualifying*

325

3.0

2.4

3.0

3.4

4.2

0.5

1.5

Null*

215

1.2

4.7

3.9

6.5

4.0

2.5

1.6

*Note that these two classifications represent
very small samples (2 cases in the non-qualifying class and 3 cases in
the null class).

Three findings
are worthy of note: (a) the composite for storms that had mid-level dry
intrusions (i.e. qualifying outbreak storms) had a lower average LCL compared
to the other two classes of storms; (b) the composite for storms that had
mid-level dry intrusions and produced outbreaks (qualifying storms) were
more moist in the layer from just above the surface through 900 hPa than
the other two classes of storms; and (c) the qualifying storms were much
drier at levels above 700 hPa than were either the non-qualifying storms
or those that had evidence of a dry intrusion but did not produce outbreaks.

It is important
to keep in mind that two of the storm classes contain very small samples.
Nevertheless, it is appropriate to observe that findings (a) and (b) are
suggestive of variations between storm classes in the depth and quality
of moisture in the lowest 100 hPa of the atmosphere. It is possible that
the “qualifying” storms were more productive because the lowest layer was
more uniformly moist than in the other two storm classes. This appears
to be consistent with the findings of Rasmussen and Blanchard (1998)12,
who (in a study of various sounding-derived supercell and tornado parameters
across the United States) noted that the LCL for soundings associated with
tornadoes was significantly lower than for soundings associated only with
supercells or even with nonsupercells. Thompson et al. (2002) report verification
of the Rasmussen and Blanchard research. Using close proximity soundings
derived from RUC-2 model hourly analyses, the authors found that substantially
lower LCLs were associated with significant tornadic supercells as compared
to nontornadic supercells. McCaul and Weisman (2002) performed simulations
of storm structure as related to variations in the depths of the mixed
layer and the moist layer by specifying that LCL=LFC in all simulations,
then varying the height of the LCL, CAPE and shear between simulations.
They found that a lowered LCL (a frequent characteristic of TC environments)
reduced the strength of surface outflow, prevented outflow dominance, and
promoted storm persistence. This potential relationship merits further
research.

Thirteen tornado
outbreak cases associated with landfalling TC along the coast of the Atlantic
and of the Gulf of Mexico occurring since 1960 have been analyzed for evidence
of mid-level dry intrusions as a factor in the outbreaks. Eleven of the
thirteen cases offer clear evidence of a dry intrusion at mid-levels over
the outbreak area. Tornadogenesis appears to be enhanced when the favorable
northeast (or right front quadrant) of the storm coincides with a pronounced
gradient in layer relative humidity, generally best reflected at
the 700 hPa or 500 hPa level.

Two distinct
patterns were identified with respect to the source of the mid-level dry
air involved in these cases: in one, a mass of dry air that impinged on
much of the northern or northwestern semi-circle of the storm's outer circulation
became divided into two lobes as the storm advanced, with one lobe to the
northwest and the other to the northeast of the storm center; the other
involved ingestion of a lobe of dry air from a reservoir most often (but
not exclusively) located in the eastern semi-circle of the storm.

The qualifying
cases were determined to have a strong diurnal signal detected in the temporal
distribution of the tornadoes, as almost two-thirds (65%) of the tornadoes
in the eleven "positive" cases occurred between 15 UTC and 00 UTC (daylight
hours in the region examined). This characteristic has been noted by other
authors, but an explanation of its relationship to tornadogenesis remains
somewhat elusive, at least in part because of the sparseness (both geographically
and temporally) of rawinsonde observations. A denser network of observation
sites coupled with more frequent sonde releases (both dropwindsonde and
rawinsonde) would be an important step for both research and operational
purposes. While modern remote sensing platforms
are invaluable aids in ascertaining the existence and extent of mid-level
dry intrusions, data from rawinsondes and dropwindsondes are critical.

Issues deserving
of additional research include determining the origin of the dry air reservoirs
that contribute to these events. Based upon the findings reported in Sec.
5.1, it is likely that there are at least two mechanisms at work, but there
may be others. As has been demonstrated herein, baroclinic surface boundaries
may play a role in enhancing the potential for tornadogenesis in
some situations. Additional research will pursue this issue through objective
analysis of surface data.

The importance
of the relationship between mid-level dry intrusions and tornado outbreaks
associated with landfalling TCs cannot be underestimated from an operational
standpoint. The dry intrusions may be detected utilizing observational
data, as demonstrated herein, but may also (at least in some cases) be
discerned from operational NCEP model output regularly available to forecasters
(Vescio et al. 1996). While not all tornado outbreaks associated with tropical
cyclone landfalls involve dry intrusions, it is clear that the recurrence
of the distinctive pattern seen in these cases should heighten forecaster
vigilance when seen operationally as a storm approaches and makes landfall.

***

12 The findings
are also consistent with updated findings reported by Rasmussen (2003).REFERENCES